Add minimal libdexfile API for external users.

This defines a stable C ABI that is safe for external use in
libdexfile_external. On top of it is a thin stable C++ API in
libdexfile_support to be linked into clients.

TBD: Unit tests, e.g. move the relevant tests from libunwindstack.

Test: mmma art/
Test: gtests and runtests on target
Test: atest system/core/libunwindstack (no new failures, 1 old one)
Bug: 119632407
Change-Id: I64388337e6d31e4bf36ec435a78c4defc2e17aa2